http://rpg.drivethrustuff.com/product/17171/D%26D-Rules-Cyclopedia-%28Basic%29?affiliate_id=10748
Hailed as the best "1 book" D&D ever it is the evolution of the BECM (no I in this one) game.
If you don't own this one now is your chance.
Everything you need to play D&D from young farm-boy or girl to Master of all you survey and plenty of challenges in between. Maps of the world of Mystara are a nice touch too.
This is the one edition of D&D I wish I had played more.